It is commonly desired to filter reservoir fluids as they are removed from underground formations. Filtration will remove particles of rock, proppants such as sand and other hard particulates deliberately introduced to the formation to assure the stability of the fissures created by fracturing, mud and the like. Underground filtration simplifies handling of the produced oil on the surface. Gravel packs are commonly used to filter downhole, in a process also known as sand control—controlling formation sand. A gravel pack is comprised of loose sand placed around the wellbore. That is, while the operator intends for the sand to reside in a compact mass, the mass is not rigid and the grains are not interconnected—they are free to move. Marginal or inferior quality gravel pack sand will contain too many fines, which clog the interstices, or will be multicrystalline, leading to crumbling and the generation of fines. Ideal gravel pack sand is spherical, but may not be available since most proppants are mined from naturally occurring deposits. Irregularly shaped grains, and wide ranges of size , also present difficulties to the operator.
The technology of expandable sand screens has also become well known in the art. See, for example, “Expandable sand screen technology increases production,” by Paul Metcalfe, World Oil, February 2000, pp 1-2, and “Using Expandable Sand Screens in Unconsolidated Formations” by Kevin McMillin, available on the internet from the Oil & Gas Journal under current petroleum, power and energy news technology. Metcalfe estimates that 40% of the world's reservoirs require sand control.
Fused or sintered filters have been devised, typically as a preformed hollow cylinder, for lowering into a well. Downhole filters prior to the present invention have been limited in their size, since the well casing is occupied by rods and must permit the passage of various devices including drill bits. For examples of pre-constructed downhole filters, see Scott, U.S. Pat. No. 5,656,176, describing a curable resin coated particulate that will form chemical bonds at contact points with other similar particulates, the particulates being formed into a well liner. Spherical particles are also said to be formed into preformed shapes designated as tubular sand screens, in Arterbury et al U.S. Pat. Nos. 5,377,750 and 5,339,895, and Nguyen et al U.S. Pat. No. 5,492,178.
The reader may also be interested in Youngman et al U.S. Pat. No. 6,372,678, which describes a ceramic proppant having an outer cured resin designed to fracture under the stress of a producing formation and an inner partially cured resin designed to cure when it is exposed to the elevated temperature of the formation; the authors say the adjacent particles are bonded into a permeable mass. Similar so-called self-consolidation particles are described by Sinclair et al U.S. Pat. No. 5,955,144. An earlier patent, U.S. Pat. No. 4,585,064 to Graham et al discloses a self-consolidating particle comprising a substrate, a substantially cured inner resin and an outer resin which may be more fully cured under the influence of the formation temperature. See also Friedman's U.S. Pat. No. 4,428,427, Graham et al U.S. Pat. Nos. 4,518,039 and 3,929,191, and Sinclair et al U.S. Pat. No. 5,422,183. Generally, these workers utilize unfinished resole, epoxy, phenol-formaldehyde, and other resins which are polymerized or cured when attaining the formation temperature. The consolidated permeable masses they form are usually located within the formation fissures, but may also be used in gravel packs. Copeland, in U.S. Pat. No. 3,857,444, uses a solvent along with the resin-coated particulate, but nevertheless relies on chemical reactions to create bridges between particles. Smith and Fullerton, in U.S. Pat. Nos. 6,030,558 and 6,399,188, describe a technique for making porous plastic products. The preferred starting materials are rapid water quenched micropellets made of polyethylene, which are fused together at a temperature of 320° F. The micropellets are diced underwater from an extrudate typically having a diameter of ⅛ inch, and are characterized by Smith and Fullerton as non-spherical.
Thermal energy has been introduced to wellbores for various purposes. Song et al, in U.S. Pat. No. 5,833,001, use electric heat to cure resin particulates in the form of a downhole sleeve. See also Gondouin U.S. Pat. No. 5,052,482 for techniques for generating steam downhole.